Whenever the fuel tank or the fuel line of an engine is exposed to cold weather, the fuel or certain impurities or additives contained in the fuel may jell or "wax", i.e., they become excessively thick. For example, the diesel fuel in the fuel tanks or fuel lines of a diesel truck may wax if the truck is parked or driven in extremely cold weather.
Since waxing can interfere with the proper flow of fuel to the engine, it frequently results in poor engine performance. For example, most fuel systems include a fuel filter mounted in the fuel line for removing impurities from the fuel. As the cold fuel flows through the fuel filter, the fuel filter may become quickly clogged by the waxed fuel. The clogged filter may limit the amount of fuel which reaches the engine and consequently cause the engine to stall. Severe waxing may even block the flow of fuel entirely, killing the engine.
Waxing may be eliminated by heating the fuel. However, while many devices for heating fuel to eliminate waxing have been proposed, they nevertheless have several undesirable characteristics. For example, many devices known as heat exchangers utilize tubes through which is passed hot exhaust gas or a coolant that has been heated by the engine. The tubes are located within or adjacent the fuel tank or the fuel line so the heat of the gas or coolant may pass through the tubes into the fuel tank or the fuel line and thereby heat the fuel. Unfortunately, it frequently requires a considerable period of time for the coolant or exhaust gas to become hot and for the hot coolant or exhaust gas to sufficiently heat the fuel in the tank or the fuel line to eliminate any waxing. Further, heat exchangers are frequently large bulky devices which are difficult to conveniently mount in the fuel system.